Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
EventBrite Online RSVP - EventBrite Online RSVP is a cost-effective event management software that provides services to help businesses organize, invite people together for an event, and sell tickets.
Eventbrite - Discover Great Events or Create Your Own & Sell Tickets
EventCreate Online RSVP - EventCreate Online RSVP is a tool for collecting RSVPs online.
Bigtent - BigTent is a web-based organization and communication portal for groups.
Magnet Event Pro - Magnet Event Pro provides tools that help you easily create a small or a large-scale event completely on your own terms.